Internal combustion engines are commonly used to provide power for motor vehicles as well as in other applications, such as for example for lawn mowers and other agricultural and landscaping equipment, power generators, pump motors, boats, planes, and the like. For a typical driving cycle of a motor vehicle, the majority of fuel consumption may occur during low-load and idling operation of the vehicle's internal combustion engine. Similarly, other uses of internal combustion engine may also be characterized by more frequent use at a power output less than that provided at a wide open throttle condition. However, due to mechanical friction, heat transfer, throttling, and other factors that can negatively impact performance, spark ignition internal combustion engines inherently have better efficiency at high loads and poorer efficiency at low loads.
Part load operation of an internal combustion engine is typically achieved by restriction of airflow into the engine via operation of a throttle. A typical throttle control mechanism also includes a mechanical or computer-controlled system (e.g. a carburetor or fuel injector system) that regulates the delivery of fuel such that a constant air/fuel ratio is maintained.
The ratio of the air mass trapped in the combustion chamber in a given engine cycle to the maximum mass of air that could be contained in the combustion chamber at its intake density is generally referred to as the volumetric efficiency. When operating under full load conditions, the volumetric efficiency of an internal combustion engine is therefore advantageously as high as possible so that the mass of the air/fuel mixture, and hence the power output, is maximized. Accordingly, an internal combustion engine is conventionally designed to minimize restriction of air flowing into the engine, so that the air can be drawn into the cylinder as close as possible to atmospheric pressure.
However, when operating at part load, the throttle restricts the airflow into the engine, intentionally reducing the volumetric efficiency to reduce output as the air pressure in the intake manifold falls significantly below atmospheric pressure. To draw air from the manifold into the cylinder, the piston must therefore do work against the lowered pressure in the manifold. This excess work done by the piston as result of the pressure differential between the manifold and the crankcase is generally referred to as a pumping loss.
As an example, many conventional internal combustion engines are typically configured for a four-stroke Otto cycle, which includes an air/fuel inlet stage, an isentropic compression stage, a constant volume combustion stage, an isentropic expansion stage, a blowdown stage, and an exhaust stage. Movement of a piston or pistons within a cylinder causes compression of a fuel mixture in a combustion volume during the compression stage to the same degree that it expands during the power stage. The Otto cycle is generally characterized as having its best efficiency at high loads with substantially reduced efficiency at lower loads (e.g. while operating a throttled condition). Pumping loses against the throttle can also be significant. The symmetry of an Otto cycle can also lead to limited efficiency. In an Otto cycle engine, a throttle is typically used to limit the airflow for part-load operation. The throttle restricts the airflow into the manifold so that the engine pulls in air from this reduced pressure region, which generally results in the work to pump the air into the engine being higher than if the valves had been used to limit the airflow.